From German Pat. No. 288041 and U.S. Pat. No. 3,677,356 it is known for such a fluid weighing device to measure the fluid upthrust on a plunger held in a tank, and also to display the weight of fluid at any level as the mathematical product of the upthrust and the ratio of the horizontal cross-sectional areas of the tank and the plunger.
Our U.S. Pat. No. 4,244,218 teaches a fluid measuring device which utilises a gyroscopic dynamometer cell and provides a digital readout of weight or volume to an exceptionally high degree of accuracy. This system is particularly attractive as the readings are taken rapidly with a full digital interface to ancillary equipment, and is insensitive to vibration and to temperature fluctuations. The upthrust from the plunger is applied to the gyroscopic load cell through a single lever which is pivoted about a fulcrum and carries a tare weight to counterbalance the full weight of the plunger. A very fine length adjuster is positioned between the plunger and the lever to ensure that the top face of the plunger can be adjusted precisely to the maximum liquid level in the tank. The callibration of the device is also described in some detail.
In our U.S. patent application Ser. No. 165,699 (continuation of application Ser. No. 22,182) we describe the manner in which the contents of particularly large liquid storage tanks can be measured using a modified fluid measuring device, and its callibration is also described in detail.
The principal method of callibration we have described in these previous Patents necessitates filling the tank with fluid, and then progressively removing the fluid in discrete steps, the fluid removed at each step being carefully collected and weighed on a second balance, and its weight being compared with the change in the weight indicated by the fluid weighing device. In several industries, for instance the chemical and petroleum industries, very large tanks are now commonly used and typically have capacities in excess of one million liters. With such large tanks, this method of callibration is impracticable as it would both necessitate having a spare tank available for receiving the fluid removed during callibration, and would involve making a very large number of callibration measurements.
It would also be conceivable to callibrate a fluid measuring device by first removing the plunger from the tank so that it is not subjected to any fluid upthrust and is counterbalanced by the tare weight. However the large tanks currently favoured by industry would require a plunger length of at least 10 meters and sometimes 20 meters or more, and it would be totally impracticable to install a fluid measuring device which could only be callibrated by incurring the very considerable time and expense of extracting such a long and heavy plunger.